Liquid crystal displays (LCDs) are used to display information by utilizing the effect that the optical birefringence of the liquid crystal layer has on the polarization of light that is transmitted through said layer. By applying a voltage across the liquid crystal layer, the orientation of the liquid crystal molecules is modified, changing the optical symmetry and hence optical retardation of the liquid crystal layer. This in turn modifies the polarization of the transmitted light. When the liquid crystal layer is positioned between two polarizing films, the change in polarization of the transmitted light is resolved into a difference in transmitted intensity. In this way, information is displayed on a liquid crystal display by spatially modulating the voltage on the liquid crystal, effectively changing individual spatial elements, or pixels, from transmitting to blocking the incident light. Liquid crystal displays are made in a multitude of configurations including transmissive, being illuminated from behind by a separate light source, and reflective, whereby the ambient light that is incident on the front of the display is reflected by a layer behind the liquid crystal and intensity resolved upon passing through the single entrance/exit polarizer on the viewing surface. Liquid crystal displays have advantages over other display media in that they can display information with much lower power consumption than emissive displays, such as plasma displays. Thus, LCDs are used in display devices such as wristwatches, pocket and personal computers, calculators, aircraft cockpit displays, etc. The very long operational life in combination with very low weight and low power consumption of LCDs, along with other design advantages, have particular utility in these applications. See U.S. Pat. No. 5,612,801, incorporated herein by reference, for an overview of LCD technology.
Liquid crystal displays exist in a multitude of embodiments depending upon the specific type of liquid crystal material and the configuration of the polarizers and electrodes. In its simplest form, a typical LCD comprises a liquid crystal cell, situated between two polarizer layers, the second polarizer layer commonly referred to as the analyzer layer. Many such liquid crystal displays exhibit optical performance that is very sensitive to the angle at which the display is viewed. Contrast, gray level separation, and color often change significantly as the viewing angle deviates from normal, or zero degrees. This viewing angle behavior arises because the optical symmetry of the liquid crystal layer is such that when the liquid crystal is electrically aligned to produce a specific transmission level at normal viewing, the angle dependence of the optical retardation through the liquid crystal layer causes the transmission at off-normal viewing to be either higher or lower than the commanded transmission at zero degrees. This can severely limit the use of LCDs in many desirable applications. Optical compensation films, also referred to as compensators or retarders, are commonly used to mitigate the viewing angle effects in LCDs, the specific design and geometry being dependent on the type of LCD employed.
A retarder, or compensator, denotes a film or plate-like birefringent optical element for which the refractive index along at least one of the optical axes is different from the other two. If the three refractive indices are different the material is called biaxial. If two of the indices are the same with the third being different the material is uniaxial. Further, a uniaxial material can have either positive or negative birefringence depending on whether the refractive index for light polarized normal to the uniaxial optical axis is less than or greater than the refractive index for light that has a polarization component parallel to the optical axis respectively.
In the ideal case, an optical compensator is designed to have an angle dependence that is complementary to that of the liquid crystal layer, thus canceling, or compensating for the angle dependence of the liquid crystal. The angle dependence is not exactly complementary because the symmetry of the liquid crystal film varies as it is electrically driven to its various transmission state orientations. The objective of a compensator design is to provide the best overall optical effect throughout the full transmission range of the liquid crystal display. The uses of such compensators have been disclosed in U.S. Pat. No. 5,196,953 (Yeh et al.), U.S. Pat. No. 5,504,603 (Winker et al.), U.S. Pat. No. 5,557,434 (Winker et al.), U.S. Pat. No. 5,589,963 (Gunning, III et al.), U.S. Pat. No. 5,619,352 (Koch et al.), U.S. Pat. No. 5,612,801 (Winker) and U.S. Pat. No. 5,638,197 (Gunning, III et al.), all of which are incorporated herein by reference.
Compensators may be placed between the polarizer and the liquid crystal cell, between the analyzer and the liquid crystal cell, or in both locations. With the addition of one or more compensator layers, the contrast ratio and grey level stability is improved over a wide range of viewing angles by careful design so the compensator, which may include multiple layers of birefringent materials each having different thicknesses, optical axis orientation with respect to the liquid crystal and polarizer angles, or sign of birefringence.
FIG. 1 depicts a coordinate system, which is used to describe the orientation of both liquid crystal and birefringent compensator optic axes. Light 116 propagates toward the viewer 100 in the positive z direction 102, which, together with the x-axis 104 and the y-axis 106, forms a right-handed coordinate system. The polar tilt angle θ (108) is defined as the angle between the liquid crystal molecular optic axis (c-axis) 110 and the x-y plane, measured from the x-y plane. The azimuthal angle φ (112) is measured from the x-axis to the projection 114 of the optic axis onto the x-y plane.
Compensator films comprise either stretched films of polymer materials, or coated films of materials having significant optical anisotropy. Stretched polymer films typically are made from polyvinyl alcohol, polystyrene, or polycarbonate. Stretched PVA films have an optical axis orientation in the stretch direction, parallel to the surface of the film, and exhibit positive birefringence. Other materials have similar in-plane optical axis orientation but with a negative birefringence. This in-plane optical symmetry is commonly referred to as “A-plate”. Stretched films are widely used but suffer from retardation non-uniformity and are affected by high temperatures which may be encountered in certain display applications. They are also limited in the range of achievable optical symmetries and therefore cannot adequately address the requirements of many high performance LCDs.
An alternative approach for LCD compensation films uses films of polymerized liquid crystals. These may be films of liquid crystal polymers that are melt cast onto a suitable substrate, which then solidify upon cooling, or films of polymerizable liquid crystal materials, commonly described as reactive mesogens, provided in a solvent and formed into films, dried, and then polymerized either using photo polymerization or thermal polymerization. The liquid crystal films may consist of nematic (rod-like) liquid crystal molecules, having positive birefringence, or discotic (disk-shaped) liquid crystals, having negative birefringence. The substrates for these films are prepared by various means to induce the desired alignment or orientation of the liquid crystal molecules in the film. The most common method for achieving this alignment is to gently rub the surface of a suitable alignment layer using a rayon cloth. Alternatively, alignment may be induced by photo alignment of a suitable photo-active alignment layer where a preferred orientation direction is imparted through the interaction with linearly polarized ultraviolet light. Further, the liquid crystal coating solution may also be modified to provide a range of optical symmetries within the liquid crystal layer, for example by the addition of a chiral component to a nematic solution to produce a twist in the orientation of the molecules in the film or to modify the orientation of the liquid crystal at the air interface.
Liquid crystal films may have a multitude of optical symmetries. For example, nematic films may be formed in the following symmetries:
For an A-plate the liquid crystal molecules are all oriented parallel to each other and substantially in the plane of the film, the direction of the long axes of the molecules determining the optical axis direction. There may be a very small tilt of a few degrees between the substrate and the first few layers of molecules, but the molecules at the air interface are oriented substantially parallel to the interface.
O-plate compensator films are formed using nematic liquid crystals, but the average value of the angle of the optical axis with respect to the surface is usually substantial, ranging from 25 to 60 degrees. The term O-plate refers to this oblique orientation of the optical axis.
O-plates may have several geometries. They may include configurations where the orientation of the optical axis is substantially constant throughout its thickness. Splayed O-plates have a varying tilt, ranging from a few degrees at the alignment surface to some steeper angle at the air interface that can be as great as 90 degrees. Finally, chiral additives may be included in the coating formulation so that the orientation of the liquid crystals develops a thickness dependent twist in combination with a constant or splayed tilt. Reference herein to “O-plates” can include all of these variations.
Discotic liquid crystal films form via the stacking of negatively birefringent disc shaped liquid crystal molecules. Discotic films occur in various configurations, including uniformly stacked discotic molecules with no tilt between the substrate and the air interface (referred to as a C-plate orientation), or alternatively, in splay configurations where the tilt of the molecules varies from a few degrees at the substrate to nearly 90 degrees at the air interface.
C-plate compensators are films having their optical axes normal to the surface. Most C-plate compensators that are commonly in use for LCDs have negative birefringence. They may include films formed using discotic liquid crystals that are aligned to have uniform orientation throughout the film thickness, films of segmented, rigid rod polyimide materials, biaxially stretched films of polystyrene, thick layers of cellulose triacetate (TAC), or multilayer structures of very thin alternating high and low refractive index materials. Negative birefringent C-plate films are useful as components in compensators for TN, VA and other display types and may be used alone or in combination with other compensation films.
An important characteristic of a liquid crystal alignment film is its ability to readily orient the liquid crystal film in a specific direction, and also to generate a “pretilt” for the aligned liquid crystal. Referring to FIG. 1, pretilt refers to a slight out-of-plane orientation of the liquid crystal molecules, typically a few degrees. For films of nematic liquid crystal materials, a pretilt of several degrees is required to insure that the liquid crystal molecules all orient with their tilt in the same direction (particularly for O-plate, splay O-plates, and twisted O-plate films). Without sufficient pretilt, a liquid crystal film will form with multiple domains and make the film ineffective for compensating a liquid crystal display. Pretilt may not be necessary for A-plates. Polyimide films are common alignment materials used with nematic liquid crystal films and are the primary material used as an alignment film for liquid crystals within the active cell of a liquid crystal display.
Most liquid crystal compensators incorporate an alignment layer coated onto a transparent substrate. Typical transparent substrates are polycarbonate or TAC. Polycarbonate can withstand higher temperatures but is expensive. TAC is less expensive and is the preferred material for most production applications. However, because the acceptable process temperature for TAC is limited by the possibility of heat deformation of the substrate, limits are imposed on the type of alignment materials and the process conditions that may be used such as drying method and temperature. Standard polyimide alignment materials typically require a high curing temperature and are therefore preferably avoided. In addition, the solvents used with most polyimide alignment layers will damage the underlying TAC substrate.